The space required by an aircraft when it is supported by the ground or a structure may be reduced by folding outer portions of the wings to be generally perpendicular to the main wing span. Folding wing-tip aircraft have been used by the military on aircraft carriers to reduce the space required by the aircraft when they are on the carrier. Since unintended folding of the wing tips during flight could be catastrophic, the known military aircraft are provided with a safety system designed to ensure that the wings are fully spread and locked into a spread condition before the aircraft takes off. The safety system relies heavily on a red flag located on each wing. A protruding red flag indicates that the wings are not properly spread.
Each wing tip of the military aircraft is hinged to the main portion of the wing and has a plurality of latch pins for locking the wing tip into a spread position. Each latch pin is provided with an inhibitor, commonly known as a "porkchop" because of its shape, that prevents extension of the latch pin into a latch position when the wing is in a folded condition. In the operation of the system, the pilot lowers a lever about half way to spread the wings. As the wings spread, lugs carried by the wing portions push the inhibitors out of their blocking position. Once the wings are spread, the pilot flips a toggle switch to latch the latch pins and then lowers the lever the remainder of its travel to lock the latch pins. Finally, he verifies that the red flag on each wing is not visible. The flags are mechanically linked to the latch pin locks and are retracted as the locks are activated. The locks are blocked from moving into their locked position when the latch pins are retracted.
In the military system, failure to achieve proper spread condition will be indicated by the flag on a wing if at least one of the inhibitors on the wing is in a nonfailed operational condition. A protruding flag is clearly visible to the flight crew and ground crew. In addition, the configuration of the aircraft makes the wing tips visible from the flight deck so that the flight crew can verify the position of the warning flags even in bad weather. Beyond the inhibitors and flags, the system is dependent on intense flight and ground crew inspections to verify that the wings are properly spread before takeoff. In an aircraft carrier environment, reliance on such inspections is practical since it is standard procedure to have a number of ground personnel checking every detail of an aircraft just prior to takeoff.
The military system has a cable-driven control path. Uncommanded fold during flight is addressed by only one level of safety. Failure of one cable can result in the latch pins becoming unlocked, thereby allowing them to migrate out of their holes. Because of the performance requirements of military aircraft, and the resulting severe space restrictions, the second or final safety level is the ejection seat.
The commercial airline industry is highly competitive, and airlines are always searching for ways to increase passenger volume and more efficiently move a given number of passengers from one airport to another. One of the main limitations on increasing passenger volume is the limited number of gates available at any given airport. This limitation has led to a trend to use larger aircraft so that each gate can service a greater number of passengers. The use of larger aircraft to increase passenger volume presents a new problem. The larger aircraft tend to have much larger wing spans than the aircraft that were commonly in use when the gates were built. In a typical gate arrangement, the larger wing span aircraft require more space than is allotted to an individual gate and, thus, the use of such aircraft would tend to reduce the number of usable passenger gates. The assignee of the present applicants has proposed the introduction of folding wing-tip commercial aircraft to solve the problem of limited space at each passenger gate so that passenger volume per gate can be increased without reducing the number of usable gates.
Experience with folding wing-tip military aircraft has provided some guidance in the design of the proposed commercial folding wing-tip aircraft. However, the usefulness of the military aircraft as a guide is limited because of the different use environments and safety requirements relating to military and commercial aircraft. It is anticipated that a commercial aircraft with folding wing tips would not move the wing tips into a spread condition until the aircraft reached the threshold of an active runway. This procedure would provide optimal use of limited width taxiways as well as limited width gate areas. Since ground personnel are not available at runway thresholds, a commercial folding wing-tip aircraft could not rely on ground crew inspections as part of safety procedures for ensuring a proper spread wing condition. In addition, the wing tips of larger commercial aircraft are not visible from the flight deck. Thus, the military warning flag system would have no practical value in the commercial aircraft environment. These considerations and the very rigorous safety requirements that apply to commercial aircraft have necessitated a search for novel approaches to the design of the various aspects of a folding wing-tip system in a commercial aircraft.